FIFO, Round-Robin Queue, Weighted RR, Fair Queue

FIFO

One potential problem of FIFO is that the small packets must wait in line for the larger packets to get dispatched.

The smallest packet is actually ready to leave before the largest packet is finished arriving. However, because the largest packet started to arrive at the interface first, it gets to leave the interface first. This actually causes gaps between data on the wire, which decreases efficiency.



Round-Robin Queue

No prioritization in RR queuing. Scheduler will cycle through all queues in a round-robin fashion, usually from top to bottom. Each queue will get an opportunity to send out a packet.

Weighted Round-Robin Queue

In WRR, packets are accessed round-robin style, but queues can be given priorities called “weights.”

Some implementations of the WRR algorithm provide prioritization by dispatching a configurable number of bytes each round rather than a number of packets. The Cisco custom queuing (CQ) mechanism is an example of this implementation.

Bear in mind, it services the queues by cycling through them in round-robin fashion, sending the portion of allocated bandwidth for each queue before moving to the next queue. If one queue is empty, the router will send packets from the next queue that has packets ready to send.

Fair Queue

Fair Queuing is a methodology that allows packets that are ready to be transmitted to leave, even if they started to arrive after another packet. (This overcomes the shortage of FIFO.) Note that FQ is not an option in Cisco routers, but understanding FQ will help you to understand WFQ.